1. Field of the Invention
The present invention relates to a synchronizing method in an optical access network, an optical switching device, a center device, and a remote device.
2. Related Art
Proposed is PON (Passive Optical Network) of an optical access network without using OSM (Optical Switching device) (for example, referring to non-patent document 1). The PON is configured with a passive element being at least one optical splitter between a center (OLT) and a remote device (ONU) with a tree shape. The PON uses the passive element, however the PON does not use the optical switching device.
The technique described in the non-patent document 1 determines a relationship between a time and a period, namely the synchronizing method, between two devices of the OLT and ONU, which constitutes the PON.
Here, there is described the relationship between the center device (OLT) in cases where there is no optical switching device (OSM) and the time concerned with transmission and reception of a packet of the remote device (ONU). The OLT and the ONU have timepieces being medium for managing the time respectively. Additionally, since there is no optical switching device, it is the same case as the non-patent document 1.
FIGS. 9A and 9B exemplify a relationship of the time concerned with the transmission and reception of the packet of the OLT and ONU in cases where there is no OSM. Additionally, two horizontal lines in FIG. 9A represent the time and the period in the OLT and ONU respectively, and FIG. 9B is an illustrative view showing a GATE message and a REPORT message indicated in FIG. 9A.
The respective dots of the horizontal line of the OLT and ONU represent the time on the timepiece of the OLT, the time on the timepiece of the ONU and distance between two dots represents the period. There are represented these time and period by adding subscripts to respective “t” and “T”. Further, a vertical direction represents distance, and spacing between two horizontal lines means distance between the OLT and certain one ONU.
In FIG. 9, first, the GATE message (also, referred to as “downstream control packet”) is made to transmit from the OLT at the time t1 on the timepiece of the OLT.
In the GATE message, described are LLID (Logical Link ID) (Identification Number ID) of the ONU of the transmission destination, Force Report R (R=0: REPORT message (also, referred to as “upstream control packet”) non-transmission, R=1: REPORT message transmission request) of Flag Field, (transmission request R of the upstream control packet), time stamp t1 (transmission time t1), grant start time t2 (transmission time t2 of the ONU), and grant length ta (transmission duration period Ta of the ONU).
For simplification-oriented presentation, hereinafter, there is indicated the ONU corresponding to the transmission destination LLID (identification number ID) of the GATE message (downstream control packet) as ONUa. In case of R=1, the ONUa sets, when having received the GATE message (downstream control packet), the time stamp t1 (transmission time t1) to the time of the timepiece of the ONUa. The ONUa transmits, when the timepiece of the ONUa indicates t2 (t2 of transmission time of the ONU) of the grant start time, the REPORT message (upstream control packet) with the length not more than Ta (Ta of transmission duration period) of the grant length. T1=t2−t1 is, in FIG. 9, determined as a system while taking into consideration of necessary period to transmit the REPORT message (upstream control packet) from the ONU has received the GATE message. Added is the time t2 on the timepiece of the ONUa as the time stamp (transmission time) in the REPORT message (upstream control packet) from the ONUa. The REPORT message transmitted from the ONUa arrives at the OLT, and its arrival time is taken to as t3 on the timepiece of the OLT.
From the process described above, the OLT calculates a round trip time RTTa between the OLT and the ONUa from RTTa=(t3−t1)−(t2−t1)=t3−t2. If there has been obtained the RTTa once, the OLT can control freely the time t3 wanted to receive from the ONU. That is, the OLT can receive the packet from the ONU at arbitrary time t3 on the timepiece of the OLT if the grant start time t2 (transmission time t2 of the ONU) written in the GATE message (downstream control packet) is taken to as t3−RTT.
If the RTTa is obtained once, it is possible to update the RTTa as the RTTa=t3′−t2, when the time of the upstream control packet received actually by the OLT is taken to as t3′. That is, the update of the RTTa is performed in every repetition of the transmission and reception of the control packet.
As found from this, it becomes important the method for obtaining the round trip time RTTa between the OLT and the ONU initially. First, there is shown explanation of the method for obtaining the RTTa, in FIG. 10.
FIG. 10 is a view showing conventional system frame, ranging window and ranging.
Generally, the OLT performs transmission control of the ONU using certain periodical frame systematically. This periodical frame is referred to as a system frame. In IEEE 802. ah of the non-patent document, the length Tf of this system frame is left to the designer of the system. At this time, the ONU before starting communication does not know the round trip time RTTa, therefore, the OLT is necessary to be considered that the ONU is distributed at the farthest position on the system design from 0 km. It is necessary to define a region with the size of Tw on the system frame when the round trip time corresponding to the ONU at the farthest position on the system design is taken to as RTTmax, length of the REPORT message using when determining the RTTa is taken to as Tg, and Tw is taken to as Tw=RTTmax+Tg. For example, if the farthest ONU from the OLT on the system design is taken to as 10 km, the round trip distance is 20 km and the Tg is little, therefore, Tw becomes about 100 μs. The range of the length Tw on the frame is referred to as ranging window, and measuring the round trip time is referred to as the ranging. When carrying out ranging initially, the ranging window may be fixed to a frame position, or it may be used variably at arbitrary position.
FIG. 10 shows an example in which fixed ranging window is placed on a position that is last position of the system frame.
First, when determining the RTTa of the ONUa, since it is unclear that the packet arrives where in the ranging window Tw, it does not cause the packet of another ONU to receive to avoid collision. The OLT issues the GATE message to the ONUa with the round trip time unclear immediately before the ranging window, the RTTa is made to determine from the REPORT message of the ONU entering into inside the ranging window Tw. If the RTTa is determined once, it is not necessary to produce the range that is inhibited to receive the packet of another ONU referred to as the ranging window, and the update of the RTTa is performed every transmission and reception described above.
However, generally, distance between the ONU and OLT, and distance between the ONU and the OSM are arbitrary, and medium for obtaining Tz cannot be given.
On the other hand, it is proposed that a tree-shaped optical network is made to configure with one optical line terminator (OLT), amplifier/splitter (AS) and plural network terminators (NT) (for instance, referring to patent document 1). Further, since medium for obtaining Tz is not given, thus connection start time of an upstream optical switching device is not given.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-295264
[Non-patent Document] IEEE Draft P802. 3ahTM/D3.3. Draft Amendment to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and physical layer specifications. Apr. 19, 2004
Next, there is described relationship of the time of an optical access network in which there is an optical switching device between the OLT and ONU.
In addition, in this case, the OLT, OSM and ONU have the timepiece respectively. FIG. 11 shows an example of the relationship of the time concerned with the transmission and reception of the packet of the OLT, OSM and ONU.
In the OSM of FIG. 11, there are obtained LLID, t1, t2 and Ta from the OLT which are written in the GATE message (downstream control packet) in the direction of ONU to find the length Tp of the GATE message (downstream control packet). The timepiece of the OSM is set to the time t1. The time when the GATE message is transmitted to the ONU from the OSM via the downstream optical switching element with the time t1 as the base point is taken to as t1+Tx.
Here, the Tx is a known value on the design of the OSM which is determined by the time to obtain LLID, t1, t2 and Tp from the above described GATE message, and delay time Ts of the downstream optical switching element. The OSM selects an outgoing port to which the ONUa corresponding to the LLID obtained from the GATE message in the downstream optical switching element is connected, a connection start time is taken to as t1+Tx−Ts, a connection duration period is taken to as Tp, and the GATE message is made to transmit at the time of t1+Tx.
The ONUa having received the GATE message passed through the OSM sets the time of the timepiece of the ONUa to t1, in the case of R=1, the REPORT message is made to produce, and the REPORT message is made to transmit when the time has reached t2. The OSM causes the REPORT message to pass and needs to transmit it to the OLT, therefore, it is necessary to determine a connection incoming port of the upstream optical switch of the OSM, a connection start time, and connection duration period to the REPORT message. For this reason, it is necessary to cause the GATE message from the OLT to correspond to the REPORT message responded thereto from the ONU in the OSM.
That is, the arrival REPORT message at the time t2+Tz−Ty corresponds to the GATE message, when a period during which the REPORT message from the ONUa gets out of the OSM is taken to as Tz, and a delay time of the upstream optical switching element is taken to as Ty, while setting the time t2 obtained from the GATE message with the OSM as the basis. Thus, the outgoing port of the upstream optical switching element can be selected from the LLID acquired from the GATE message, and there are obtained the connection start time with t2+Tz−Ty, and the connection duration period from Ta.
Meanwhile, in the technique described in the non-patent document 1 described above, Ty is the value known beforehand on the design of the OSM, t2 and Ta are the values obtained from the GATE message. Therefore, switching control of the upstream optical switching element in the OSM becomes possible if Tz is known. Problem is how to obtain this Tz in the OSM.
Further, the patent document 1 has not described the point of using the optical switch medium OSM.
Consequently, in order to achieve the above mentioned problem, it is an object of the present invention to provide a synchronizing method in an optical access network capable of giving a connection start time of an upstream optical switching element of an optical switching device, an optical switching device, a center device, a remote device, an optical access system, an optical access network, a program, and a recording medium.